If you have the August copy of PGI, Robbo goes over this at the back.
If not, as mentioned, a good paint/batrrel match is important.
The next thing is the consistency of your air system...CO2 is not consistent...a good quality regulator on your Airsystem will give consistently similar velocities, which means that the paintball will hit much the same place, theoretically.....basically, if you have a full chamber of air before you pull the trigger, the paintball will go a certain distance...next time it might have only 80% of the previous volume in the chamber, so less air to push the ball, the ball goes a shorter distance. Then you have wind etc to take in to consideration, but that comes with experience.
Dont go thinkng you need a long barrel. You only need 6 inches of barrel to accellerate a paintball, the rest of it is make it quiet.
If the barrel is too long it will need more air to push the ball up at the required velocity, if it has lots of pretty holes drilled in it, this has the same effect-tho makes the shot quieter.
You can test this by chrono-ing a marker with a long barrel, then stick a short barrel on and see the velocity go up.
so: Good paint
Good fit between barrel and paint
Good consistent air system
Good barrel.
The only side to this is Tipmanns flatline barrel which is bent in order to impart backspin on the ball, making it travel further.
Hope this helps.
Gaz